This invention relates to a variable gain amplifier incorporating a resistor ladder circuit with a linear-in-dB transfer function, to provide an output with linear-in-dB gain steps.
In many electronic applications it is necessary or preferred to be able to adjust signal levels in steps that are linear when measured in decibels, or “linear-in-dB.” Because intensity in decibels is a logarithmic function, this means that circuits that act logarithmically, or can mimic logarithmic activity, are desirable.
Bipolar transistors, by the exponential nature of the physics of their operation, are inherently logarithmic in operation. However, most electronic devices are now integrated devices that are not inherently logarithmic. Thus, various techniques are used to create or approximate linear-in-dB output from such devices. For example, resistive ladders can be constructed, in which any resistor can serve as the input tap, providing different outputs. By choosing particular resistor values, transfer functions that are linear-in-dB can be obtained or at least approximated.
However, there is no regular, rational relationship among the values of the resistors in the ladder. The values simply have to be calculated, practically by trial-and-error, for each application. Even then, the result may only approximate linear-in-dB operation.
In another approach, a variable gain amplifier—e.g., using a current mirror—can be constructed, with a multi-bit control input to create a transfer function with many steps. For example, with a 10-bit control signal, 210 steps can be created. Of the 1,024 steps of the resulting transfer function, the designer can then select—essentially by hand—those steps that, taken together, mimic linear-in-dB behavior. The other steps remain unused. This approach therefore requires significant overhead in unused steps to obtain enough steps to approximate linear-in-dB behavior.
It would be desirable to be able to provide a variable gain amplifier that provides a true linear-in-dB output with little or no unnecessary overhead.